Learn Schema Validation With a Tiny GitHub Issue Fields Project

GitHub announced on July 2, 2026 that Issue fields are generally available, including integration with GitHub's MCP server. Primary source: GitHub Changelog, July 2, 2026 . That creates a useful beginner project: validate structured issue metadata before an MCP client—or any program—uses it. The schema below is invented for learning. It is not GitHub's API schema. Define three fields // schema.mjs export const schema = { priority : { kind : " singleSelect " , required : true , options : [ " P0 " , " P1 " , " P2 " , " P3 " ] }, estimate : { kind : " number " , required : false , min : 0 , max : 100 }, customerImpact : { kind : " text " , required : false , maxLength : 120 } }; A schema describes both type and domain rules. estimate must be a number, but it also has to fit the range this project accepts. Validate at the boundary // validate.mjs import { schema } from " ./schema.mjs " ; export function validate ( input ) { const errors = []; if ( ! input || Array . isArray ( input ) || typeof input !== " object " ) { return [ " fields must be an object " ]; } for ( const [ name , rule ] of Object . entries ( schema )) { if ( rule . required && ! ( name in input )) errors . push ( ` ${ name } : missing` ); } for ( const [ name , value ] of Object . entries ( input )) { const rule = schema [ name ]; if ( ! rule ) { errors . push ( ` ${ name } : unknown field` ); continue ; } if ( rule . kind === " singleSelect " && ( typeof value !== " string " || ! rule . options . includes ( value ))) { errors . push ( ` ${ name } : expected ${ rule . options . join ( " , " )} ` ); } if ( rule . kind === " number " && ( typeof value !== " number " || ! Number . isFinite ( value ) || value < rule . min || value > rule . max )) { errors . push ( ` ${ name } : expected ${ rule . min } .. ${ rule . max } ` ); } if ( rule . kind === " text " && ( typeof value !== " string " || value . length > rule . maxLength )) { errors . push ( ` ${ name } : expected at most ${ rule . maxLength } charact